1. Field of the Invention
The invention relates to a magnetic resonance device for determining a nuclear magnetization distribution in a region of an object, which device comprises
(a) means for generating a steady, uniform magnetic field, PA1 (b) means for generating RF electromagnetic radiation in order to generate magnetic resonance signals; PA1 (c) means for generating at least one gradient magnetic field with an adjustable gradient direction in order to influence the magnetic resonance signals, PA1 (d) a detection chain comprising a signal amplifier for detecting, amplifying and sampling the resonance signal during a measuring period, and an analog-to-digital converter which is connected thereto, PA1 (e) processing means for processing the sampled resonance signals, and PA1 (f) control means for controlling at least the means specified sub (b) to (e), resonance signals being conditioned during a preparation period preceding the measuring period, the control means supplying the means specified sub (c) with control signals for adjusting the strength and for the duration of at least one gradient magnetic field such that the integral of the strength of the gradient magnetic field over the preparation period is different, for each conditioned resonance signal to be sampled.
2. Description of the Prior Art
A device of this kind is known from European Patent Application publication No. 0 155 052 corresponding to U.S. Pat. No. 4,682,110 and from British Application No. 2,177,861 which corresponds to U.S. Pat. No. 4,700,138. In such a magnetic resonance device an N-dimensional data set is recorded, producing the desired image or spectrum after an N-dimensional Fourier transformation. Subsequent to an excitation pulse, various so-called spin echoes can be produced by means of 180.degree. echo pulses or by continuously switching over the measuring gradient field after each measuring period, each of said spin echoes being and producing a set of data for said N-dimensional data set. Because of gradient magnetic fields applied during a preparation period and because of T2 relaxations and/or field inhomogeneities, the intensity of the various resonance signals will differ. The greater the influence of the magnetic gradient fields, field inhomogeneities and the longer the time during which the T2 relaxations can occur, the weaker the magnetic resonance signals will be. In a conventional magnetic resonance system in which a nuclear spin density distribution is determined, a set of data having an index n determined by the time integral over the preparatory gradient magnetic field is obtained by sampling a resonance signal influenced by a preparatory gradient magnetic field during a preparation period. During the successive measuring cycles this integral should have different values which form an arithmetical series n.A. The value of n then ranges, for example from -127, via 0 to +128 when an image comprises 256 pixels in a direction defined by the gradient of the preparatory gradient magnetic field. The signal to be sampled for the data set O (no influencing by a preparatory gradient magnetic field) will have the highest amplitude. The signal amplitudes of the signals to be sampled will decrease very quickly, notably in the present case, as a function of the data set index n (certainly for .vertline.n.vertline.&gt;2). Typically, for the images to be reconstructed the strongest signals are concentrated in the central five data sets (-2&lt;n&lt;2) and the signal level for the remaining data sets will be at least a factor ten lower. The adjusting means are used for adapting the receiver chain to varying signal strengths over echo resonance signals for different MR images in the case of multiple echo or multiple-slice pulse sequences. Within an image the grain remains constant.
A publication by R. Ernst in Journal of Magnetic Resonance, Vol. 4, 1971, pp. 280-296 states that the effect of the quantization noise (the noise added to a signal upon its conversion from analog to digital form) is negligible when the noise level of the signal on the input of the analog-to-digital converter is effectively at least equal to the least significant bit (=quantization step). When an analog signal is too strong, its amplitude will have to be adapted to the range of an analog-to-digital converter, implying an attenuation of the signal. As a result, the analog-to-digital converter adds noise in the form of quantization noise to the signal. The foregoing can be avoided by taking steps which limit the signal dynamics but which have the drawback that they require additional hardware or additional signal processing. In a magnetic resonance apparatus the receiver is switched over to a lower gain or a higher gain during acquisition of successive echoes in order to drive the analog-to-digital converters to full output. The noise factor of the overall receiver will increase as the receiver gain factor decreases. The signal-to-noise ratio of the images will then be a function of the setting of the gain of the receiver. In the above case the signal-to-noise ratio will be far from optimum.
In the case of a low gain factor of the receiver which will be selected notably for forming a 3-dimensional image and/or for the imaging of thick slices, the noise factor will be increased by several dB, so that a loss mounting to tens of percents occurs in the signal-to-noise ratio. The foregoing is a rather universal problem which is becoming more serious because of the increased strength of the signals to be processed due to improvements in the magnetic resonance receiver coils.